The present invention relates, in general, to a pneumatic gate valve, and more particularly to a pneumatic valve gate having a valve housing, which is made of injection-molded plastic material, and a slide member, which is axially movable in a valve bore of the valve housing, for regulating a flow of compressed air between several external ports of the valve housing.
In a pneumatic valve gate of this type, the valve bore is formed by a plurality of valve bushings which are disposed in the interior of the valve housing in axial spaced-apart relationship, thereby defining pressure fluid compartments in fluid communication with the outer ports. The valve housing can be composed of several parts or may be made in single-piece construction. Split valve housings are generally used in order to simplify the installation of components to be accommodated in the valve housing. Usually, the multi-part valve housing is split in a plane of the slide member. After mounting the internal valve components, the housing parts are connected together by a screw connection with interposed seal. Pneumatic gate valves with split valve housing are, however, rather bulky as the contact surface for the seal requires a certain material thickness.
There is an ever increasing demand on the manufacturers to produce pneumatic gate valves of smaller size because the provision of small single valves enables the production of an overall compact valve unit composed of a multiplicity of adjoining single valves. Therefore, the use of a single-piece valve housing is more and more desired as the overall width can hereby significantly be reduced, without causing an excessive decrease in throughflow of pressure fluid. On the other hand, single-part valve housings must be refinished by a complicated material cutting process, after their production through injection molding, especially as far as the interior valve bore for accommodation of the slide member is concerned. The finishing process of the valve bore involves the provision of a particularly smooth surface and the formation of various slanted inlet ramp surfaces and rounded control edges.
German patent publication DE 197 45 802 A1 describes a single-part valve housing including individual, pre-fabricated valve bushings which form the valve bore and are molded around with plastic material as the valve housing is injection molded. In order to secure the valve bushings in axial direction, each valve bushing is provided on its outer circumference with at least one ring-shaped groove or, at least in predetermined sections, with a ring-shaped bump, to effect a form-fitting engagement with the valve housing, after the valve housing is molded around the valve bushings. The valve bushings, which are made of metallic material, are placed into the injection mold before the injection molding process and held in place at the appropriate positions by rams. After injection molding, the valve bushings are secured in form-fitting relationship with the valve housing.
Although this approach may appear to be sound, practice has shown that when a gate valve of this type is used over an extended period or exposed to extreme temperature fluctuations, a clearance between the valve bushings and the valve housing may form that adversely affects the operation of the gate valve. In extreme situations, the emergence of a gap may lead to an inadvertent flow of pressure fluid between neighboring pressure fluid compartments. To tackle this problem, the arrangement of a sealing element in the form of an O-ring between the valve bushing and the valve housing has been proposed. However, as plastic material is molded around the valve bushings as the valve housing is injection-molded, the sealing elements are pushed out of the intended position so that the valve housing cannot be manufactured at reliable reproducibility.
It would therefore be desirable and advantageous to provide an improved pneumatic gate valve which obviates prior art shortcomings and which is reliable in operation even when subjected to extreme stress, while still allowing a reproducible production of the valve housing on large scale with constant quality.